To prevent global warming, the reduction of CO2 emission has recently been required, so that improvements in fuel efficiency and combustion efficiency in automobile engines have been pursued. However, improvement in combustion efficiency by elevating the combustion temperature or by direct injection of a fuel into the combustion chamber of an engine leads to the degradation of an engine-lubricating oil, so that the resultant oil sludge makes side rails stick to a spacer expander, wearing seating tabs of the spacer expander.
To prevent the sticking of a spacer expander to side rails with oil sludge, for example, JP 2000-27995 A proposes the formation of a fluorine-containing, oil-repellent coating on a surface of the oil ring, the coating being formed by a sol-gel method from a metal alkoxide and fluoroalkyl-substituted metal alkoxide, in which part of alkoxyl groups are substituted by fluoroalkyl groups. Also, to prevent the seating tabs of a spacer expander from being worn, JP 56-66429 A proposes the soft-nitriding treatment of seating tabs of a spacer expander, and WO 2005/040645 A proposes that a gas-nitrided layer containing an S phase having peaks at 2θ of 40° and 46° in X-ray diffraction with Cu—Kα is formed on seating tabs.
However, the formation of a nitride layer on seating tabs of a spacer expander makes two upper and lower side rails easily rotatable in a circumferential direction separately. If gaps of the upper and lower side rails overlapped, a lubricating oil remaining in the gaps would be conveyed into combustion chambers of an engine, resulting in excessive supply of the lubricating oil and increased oil consumption.
As measures for preventing the separate rotation of side rails, surface roughening for increasing a friction coefficient, and the formation of projections on the side rail-pushing surfaces of seating tabs of a spacer expander to increase a pushing force per a unit area, thereby increasing friction resistance, has been proposed. For example, Japanese Utility Model Application Publication 1-78768 A proposes the formation of fine projections and recesses, for example, saw-like projections and recesses having a pitch of 25-250 μm and height of 15-180 μm, on seating tab surfaces, to increase friction resistance to prevent the rotation of side rails. JP 2001-132840 A proposes the formation of projections as thick as 0.07-0.2 mm in center portions of seating tabs to reduce a contact area, thereby increasing the contact surface pressure of the seating tabs to prevent the rotation of side rails. Further, JP 2013-87816 A proposes the formation of longitudinal projections and recesses having a substantially trapezoidal waveform cross section, on the side rail-pushing surfaces of seating tabs of a spacer expander, taking into consideration fretting fatigue due to the swinging of a piston.
However, when a nitride layer is formed on the seating tabs of a spacer expander, the above measures for preventing the separate rotation of side rails may not work well. It has not been found yet what is necessary for surely preventing the separate rotation of side rails.